Microwave - Reasons for the Exterior Case or Interior Floor to Feel Hot
Cooking foods in a microwave oven for an extended period of time can cause heat to transfer to the internal oven walls and floor as well as the external cabinet. In addition to the heat that is generated outward from the food being cooked, the oven interior lamp and the magnetron tube will also contribute to external heat transfer. This condition is considered normal. The exterior case may have a symbol that looks like a triangle with waves on it stamped in the metal. This symbol means "Caution, hot surface".
It should be noted that internal and external temperatures after extended cooking will vary based upon the type of food prepared and the length of the cooking time.
With Over-the-Range microwaves, if the cooktop lights are left on full power for prolonged periods, the oven cavity floor will become hot to the touch.
The floor of the microwave interior can be very hot after popping popcorn.